The I region of the major histocompatibility complex contains immune response genes that display considerable polymorphism; that is, there are many alleles at each locus. These genes regulate the immune response to antigen by mediating intercellular communication among lymphoreticular cells. An analysis of the primary structure of the products of two subregions of (I-A, I-E/C) was undertaken in order to understand the genetic organization of the region, the evolution of the genes and, eventually, their function.